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The two small circuits
below are simple pre-amplifiers that can easily be assembled to be used
with many of your intended audio projects . Any similar transistors
can be used. The RF potentiometer adjust the negative feed back used
to bias to Q1 while Q2 provides for a low impedance output.
Both circuits require a minimum input of 100mV
for a gain of 20 for A at 14 mA and a gain of 30 for B at 26 mA into
a 5K ohms load without distortion.
If you are in need of something simple to check the presence of
low DC voltage on your projects the next two circuits give you a
choice of a buzzer (figure C) or an LED (figure B) as an indicator
when voltage is detected.
Circuit " G " is a little gimmick that you might like to try.
It is a remote control switch that can be activated only by the
direct beam of a flashlight or lazer. It could be installed hidden
in a dark area where you would prefer not to have a visible wall
switch to activate anything you like. The phototransistor is mounted
into the recess of a small tube deep enought and calibrated so
that ambient light would not activate it.
The next circuit , figure " H " is a DC voltage level detector
. It is straight forward in its application, adJusting the 100K
potentiometer for the voltage level to be detected will activate
the relay which in turn can turn on or off another devices such
as a lamp, switch,circuit .
© Laurier Gendron, Burnaby, B.C., Canada. 1998